The present invention relates to a process and a circuit arrangement for measuring the resistance of a resistance sensor, particularly a wetness sensor, via the rate of change of the charge of a capacitor, one measurement cycle in each case consisting of a charging of the capacitor followed by an incomplete discharging of the capacitor over the resistance sensor, and then measurement of charge remaining in the capacitor.
As an example, in order to measure the wetness on the windshield of a motor vehicle, sensor resistors in the form of two strip-shaped electrodes which are interlaced in each other are used. If drops of water contact both electrodes, then the resistance decreases. The change in resistance is used for the automatic control of a windshield wiper. In order to measure the resistance, it is necessary, in principle, to apply a voltage to the resistance sensor. Due to dc-voltage portions of the voltage to be applied, electrolysis phenomena may however occur between the electrodes, leading finally to a decrease in the life of the resistance sensors. A device for controlling a drive means for an automobile accessory having such a resistance sensor is described, for instance, in WO 90/08680.